After a two-week hiatus in aerobraking, NASA's Mars Global
Surveyor flight team will resume lowering the spacecraft's orbit
beginning Nov. 7. The effort will proceed at a more gradual pace
than before, which will extend the aerobraking phase by eight to
12 months and will change Global Surveyor's final mapping orbit.

The decision to resume aerobraking came after intensive
engineering analysis, computer simulations and tests with
representative hardware to characterize the current condition of
one of the spacecraft's two solar panels, which began to flex
more than expected during the spacecraft's lowest dip into the
Martian atmosphere on Oct. 6.

Under normal circumstances, the spacecraft's two 3.5-meter-
long (11-foot) solar panels should remain fixed and nearly
motionless during each aerobraking pass through the upper
atmosphere of Mars. One of the panels, which did not fully deploy
and latch after launch, moved past its latched position and has
shown slight movement during the spacecraft's last three closest
approaches to the Martian surface.

"After sufficient time to study the observed motion, we
concurred that it is possible to perform additional aerobraking
at a slower rate, without putting undue stress on the solar panel
in question," said Glenn E. Cunningham, Mars Global Surveyor
mission manager at NASA's Jet Propulsion Laboratory, Pasadena,
CA. "This changes Mars Global Surveyor's final mapping orbit,
but it should not have a significant impact on the ability of
Global Surveyor to accomplish the mission science objectives."

The spacecraft's scientific instruments have performed
flawlessly and continue to return new information about Martian
magnetic properties, its atmosphere, surface features,
temperatures and mineralogy since Mars Global Surveyor entered
orbit around the red planet on Sept. 11.

The spacecraft is currently in a 35-hour elliptical orbit
which brings it 172 kilometers (107 miles) above the surface of
Mars at its closest approach to the planet. The operations team
at JPL and Lockheed Martin Astronautics, Denver, CO, will begin
to reduce that orbit using a more moderate level of aerobraking
that will slowly bring the spacecraft into the desired nearly
circular mapping orbit. Aerobraking, a technique first
demonstrated in the summer of 1993 during the final months of the
Magellan mission to Venus, allows a spacecraft to lower its orbit
without relying on propellant, by using the drag produced by a
planet's atmosphere.

"There are several other desirable orbits for us to consider
in the next several weeks that will give us global coverage of
the planet and yield all of the science data we expected to
return," Cunningham said. "In the meantime, the instruments are
performing marvelously, and we will continue gathering new
science data as we begin to reduce the spacecraft's altitude and
bring it down into the upper Martian atmosphere. Even if we wind
up in an elliptical orbit, we will have an opportunity to study
Mars at closer range than we originally planned because the
spacecraft's periapsis -- or closest passage over Mars -- will be
closer than the 378-kilometer (234-mile) circular orbit that was
to be its original mapping distance."

The spacecraft's current orbit was raised Oct. 12 after the
flight operations team observed that the unlatched solar panel
had moved more than 20 degrees and beyond what should have been
its fully deployed and latched position. Significant movement was
observed on periapsis 15 -- or the 15th closest pass over Mars,
which occurred on Oct. 6 -- when the Martian atmosphere had
become twice as dense as it had been during previous passes. The
thickness of the atmosphere amounted to a 50 percent increase in
pressure over what was expected on the spacecraft's solar array.

Although atmospheric variations like these were anticipated
as the seasons change on Mars, the spacecraft's orbit was raised
by about 11 kilometers (7 miles) to adjust the pressure level.
Subsequent motion of the panel at periapsis 16 though 18 caused
the flight team to raise the orbit further on Oct. 12, taking the
spacecraft out of the atmosphere altogether.

"The investigation of the unexpected motion of the unlatched
panel led us to identify a secondary source of damage in the
yoke, a piece of structure that connects the solar panel to the
spacecraft," Cunningham said. "This secondary source of damage
was a result of the failure of the damper arm that jammed in the
panel's hinge joint shortly after launch when the solar panels
were initially deployed."

Mechanical stress analysis tests suggest that the yoke -- a
triangular, aluminum honeycomb material sandwiched between two
sheets of graphite epoxy -- probably fractured on one surface.
The analysis further suggests that the fractured surface, with
increased pressure on the panel during aerobraking, began to pull
away from the aluminum honeycomb beneath it.

"Aerobraking will be reinitiated at 0.2 newtons per square
meter (3/100,000ths of 1 pound per square inch), which is about
one-third of the original aerobraking level," Cunningham said.
"This is a pressure that we currently believe is safe but we will
continue to work with ground tests, analysis and close monitoring
of in-flight spacecraft data to assure that it is safe," he
added.

"Aerobraking will take much longer, perhaps eight to 12
months, at this more gradual rate. In the meantime, we will
continue collecting science data and work in the next several
weeks toward selection of the best possible orbit to fulfill the
science objectives of the mapping mission."

Mars Global Surveyor is part of a sustained program of Mars
exploration known as the Mars Surveyor Program. The mission is
managed by the Jet Propulsion Laboratory for NASA's Office of
Space Science, Washington, DC. JPL's industrial partner is
Lockheed Martin Astronautics, Denver, CO, which developed and
operates the spacecraft. JPL is a division of the California
Institute of Technology, Pasadena, CA.